Mojo SL fitting issue

I have this issue with my mojo SL, not sure if its because I am border line between medium and large, I went to a fitting to my LBS and they raised the seat post way high, I was complaining because I had a sharp knee pain on my right leg.

The guy who fitted me said that I need it to order a new seatpost because they one I had was short for the size that I need, and yes the pain started to go away little by little with the taller seatpost, its still there but not as it was before, could changing my bike to a large help ease the pain on my knee...